The purpose of this test is to evaluate the correct operation of the Bosch CDi quantity control valve based on the voltage and duty control under engine run conditions.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
The example waveform illustrates that the fuel quantity control valve is actuated by a switched-Earth, pulse-width modulated (PWM), voltage:
The signal has a fixed cyclic period in which its voltage switches between battery positive (at around 15 Volts) and close to battery negative (at just above 0 Volts).
The period the signal spends around 0 Volts relative to the total cycle period indicates the duty of the PWM signal. In this example, approximately two-fifths of each cycle is spent close to 0 Volts, so the PWM duty is about 40%.
Nearly (just under) one cycle is completed every 5ms, indicating that the signal has a base frequency equal to just under 200 Hz (185 Hz).
Within a common rail diesel system, the engine control unit uses a quantity control valve to regulate the fuel pressure entering the common rail via the high-pressure pump: when the engine control unit needs to reduce the pressure, the valve is closed and less fuel is drawn through the pumping mechanism (excess fuel is released to the fuel return system). Conversely, when an increased pressure is needed, the valve is opened and an increased quantity of fuel is delivered through the pump.
A quantity control valve’s position is determined by the action of a solenoid against a spring. With these devices, the valve will move from its default position (which may be either open or closed, depending on the application) when current flows through the solenoid. The greater the current, the greater the displacement of the valve. Thus, in some systems an increase in current will cause the valve to become more closed, whereas in others it will cause the valve to become more open.
An engine control unit can efficiently control current in a circuit using a pulse-width modulated (PWM) signal and, for a given electrical load, the greater the duty period (the relative amount of ‘ON’ time relative to the whole cycle), the greater the average current flowing through the circuit.
With a switched-Earth activated circuit, the solenoid is fed with a constant battery positive feed on one side and, on the other side, the engine control unit modulates the path to the battery Earth, to create current flow. Therefore, in these circuits, the valve is energised (ON) when the actuation signal voltage is at battery negative voltage and de-energised (OFF) when the actuation signal voltage is at battery positive voltage. Hence the greater the duty cycle, the greater the current in the circuit and the greater the displacement of the valve from its default (open or closed) position.
The engine control module will vary the duty depending on the engine speed, load and temperature conditions and the torque demand from the driver (accelerator pedal position).
By reducing the quantity of fuel entering the high-pressure pump and common-rail system, the loads on the pump and engine, are reduced, helping to increase the efficiency of the engine.
Refer to vehicle technical data for specific test conditions and results.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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